Tool/software:
Hi,
I have a customer who is experiencing failures on USB interface. On boards they received back from the field after failing, they have found that the device fails to enumerate. They believe Vbus must not be detected and no enumeration happens.
I have attached further detail below:
What they have discovered in debugging is that after the failure it seems that there is no device Mode pullup on D+ (should be ~1.5kOhm). Both D+ and D- are sitting at ground potential. That might be either they are disconnected, or the USB interface tries to enter Host Mode (in this case both D+ and D- are pulled down with ~15kOhm). There is +5V VBus on the top layer trace to the MCU VBus sense pin. The 1.5kOhm pullup on D+ signals to the Host that a Device is on the bus so the Host could start enumeration and assign address and whatever else to the device. No pullup, no device as far as the Host is concerned so enumeration won't start. Sure enough, the USB Device on the MCU side is not configured with anything but reset values except the SOFTCONN bit is set. That bit pulls D+/D- pins out of 3-state so when the interface is in the Device Mode (it is, the DEV bit in USBDEVCTL is set on power-up so it defaults to Device Mode initially and it is still set after USB init by the RTOS) so it should put a pullup on D+. It doesn't do this, D+ is at ground.
Power cycle does not fix the issue or reflashing the MCU. It is like the USB PHY is broken.
They suspect it may be due to B16 (USBVBUS) tied directly to VBus - the voltage is around 5.2V. Would 5.2V on B16 cause damage to the system?
I also can share the schematic via email, please let me know if this is needed.
